Various harmful components are included in an exhaust gas discharged from an internal combustion engine used for automobiles, etc., or an incineration system. Among the harmful components, it is particularly desirable to reduce NOx, SOx and PM emissions. Regarding diesel engines, it has become desirable to reduce PM, and a particulate filter has been used to remove PM.
Such a particulate filter has a porous honeycomb structure made of ceramics such as cordierite, etc. Inlet openings of passages compartmentalized in a grid form are alternately plugged, while the remaining passages whose inlet openings are not plugged are plugged at their outlet openings. Consequently, the exhaust gas passing through porous walls which define the passages can be discharged toward the downstream side. PM is captured on the inner surfaces of the porous walls because PM in the exhaust gas cannot pass through the porous walls.
Such a filter is clogged with captured PM, causing an increase of flow resistance followed by a burden on an engine. Therefore, it is necessary to regenerate the filter by appropriately burning and removing PM prior to the increase of flow resistance caused by clogging. Under the operating conditions of a normal diesel engine, however, the exhaust gas does not reach a temperature sufficient for the self-burning of PM.
Subsequently, an exhaust gas purifying device (plasma reactor) has been recently proposed in which plasma is generated by a discharge, PM is burnt and removed by the oxidation action of the plasma, and NOx or the like is cleaned by the oxidation action of the plasma and the reduction action of a catalyst.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2001-162134 proposes an exhaust gas purifying system which comprises a circular cylindrical outer circumference electrode, a discharge metal wire electrode at the center part of the outer circumference electrode, and an electrically insulated hollow cylindrical body inside the outer circumference electrode. In the system, the outer circumference electrode is grounded, the metal wire electrode is connected to a power supply source, a discharge is generated between the outer circumference electrode and the metal wire electrode by the action of the power supply source to generate plasma, and an electrically insulated hollow cylindrical body is arranged between the outer circumference electrode and the metal wire electrode to avoid useless energy consumption of sparks.
However, in the above-mentioned exhaust gas purifying device, a large amount of energy is required to generate plasma, and therefore, there is room for improvement in energy efficiency. An object of the present invention is to provide an exhaust gas purifying device having an improved PM capture rate, etc., as well as energy efficiency to solve the problems.